Nepises sice aky mas OS, respektive o aku verziu Outlooku sa jedna, ale skus este toto:

Reset Your Default Programs

In Internet Explorer, Select Tools > Internet Options.
Select Programs > Set programs.
Select Set your default programs.
In the Programs list, select Microsoft Outlook > Set this program as default.
Scroll back up to the top of the default programs list and highlight Internet Explorer and select Set this program as default.
Select OK, and close the Default Programs dialog box

a keby nepomohlo tak este uprava registrov.

Select Start > Run.
In the Open box, type regedit, and then select OK.
Locate, and then click the following registry subkey: HKEY_CLASSES_ROOT \.html
Make sure that the value of the String (Default) is “htmlfile“. If it’s not “htmlfile”, right-click (Default), select Modify, input htmlfile in the Value data box, and then select OK.